1. Field of the Invention
This invention relates to lasers and more particularly, to the use of lasers in optical time domain reflectometers (OTDRs).
2. Description of the Prior Art
The telecommunications industry is making increasing use of optical fiber to provide both subscriber and long distance services. Several companies have installed optical fiber networks to provide service between a number of cities located in different states. The rates at which information is transmitted on such networks has steadily increased as has the use of such networks. A disruption in service caused by a break in the fiber, whether accidental or intentional, can then have substantial adverse economic consequences not only for the company providing the service but also for the users of the service. Therefore, it is desirable that the service providing company be able to quickly and easily locate the site of the break so that the same can be repaired.
The service providers have been using OTDRs in order to locate the site of the break. In my U.S. patent application Ser. No. 07/264,356, which was filed on Oct. 31, 1988 and is assigned to the same assignee as is the present invention (hereinafter "the '356 application"), now U.S. Pat. No. 4,958,926 which issued on Sept. 25, 1990, there is described an OTDR which can be permanently connected to an optical fiber. The OTDR of the '356 application includes closed loop circuitry which allows the OTDR to continuously monitor the light backscattered from the fiber to which it is connected in response to a pulse of light from the laser of the OTDR. When there is a break in that fiber, the OTDR display is used to determine the location of the place at which the break has occurred.
The length of optical fiber span, i.e. distance D in Km, that can be monitored by the OTDR of the '356 application can be calculated as follows: EQU D=DR/2a (1)
where DR is the dynamic range, i.e. how far the OTDR can see and .alpha. is the fiber loss coefficient in dB/Km. For an optical fiber having a 10 micrometer core and a 125 micrometer cladding the loss coefficient is about 0.25 dB/Km.
The dynamic range can be calculated as follows: EQU DR=Ps-2Lc-2Ls+P.sub.B -P.sub.R -Margin (2)
where Ps is the power of the laser source in dBm, Lc is the splitting loss of the coupler by which the laser of the OTDR is connected to the fiber, Ls is the excess loss (coupler, connector, etc.), P.sub.B is the backscatter ratio in dB, P.sub.R is the OTDR receiver noise equivalent power including the averaging and processing improvement in dBm and Margin is a predetermined design safety factor.
For example, in an OTDR of the type described in the '356 application wherein the laser diode operates at a wavelength of 1550 nanometers and provides a pulse of light having a width of one microsecond and the OTDR has the following parameters:
Ps=10 dBm PA1 Lc=3 dB PA1 P.sub.B =-53 dB PA1 P.sub.R =-95 dBm (a receiver with a one microsecond time resolution, a noise equivalent power of -70 dBm and a processing improvement of 25 dB); PA1 DR=41 dB. With a loss coefficient of 0.25 dB/Km for the optical fiber, the distance as calculated from equation (1) is: PA1 D=82 Km (about 51 miles).
In applications where long repeatered spans of transmission optical fiber are to be monitored, it is advantageous to increase the length of fiber being monitored by each OTDR. As can be seen from equation (1) the distance that the OTDR can monitor is directly proportional to the dynamic range. In the OTDR of the above example, in order to double the distance the dynamic range would also have to be doubled, i.e. the dynamic range would have to be increased to 82 dB.
For a doubling of the dynamic range and the same receiver sensitivity (this assumes that the receiver sensitivity has already been maximized), it can then be determined from equation (2) that Ps would have to be increased to 51 dBm, i.e. an increase of 41 dBm. That is over a 12,000 fold increase in laser power. In other words, the power would have to be increased from the 10 milliwatts used in the laser of the OTDR of the '356 application to more than 120 watts. Such a laser is not commercially available today. In addition, at such power levels the response of the optical fiber of the type described above becomes nonlinear and it is not possible to monitor the fiber using the backscattered light.
Even a smaller increase in dynamic range may still require a substantial increase in laser power, as for every 5 dB increase in dynamic range the laser power must be increased by a factor of 10. For example, an increase in dynamic range by 15 dB would require a 1,000 fold increase in laser power from 10 milliwatts to 10 watts. Therefore, it is clear that substantial increases in laser power are needed to obtain even relatively modest increases in distance.
In a manner similar to the above examples, it can be shown that a doubling of the laser power for the same receiver sensitivity would only increase the distance by a very small amount. The reverse is also true. It can also be shown that decreasing the laser power by half for the same sensitivity would only diminish the distance by a small amount.
The object of the present invention is to nearly double the distance of optical fiber that can be monitored with the same laser power and the same receiver sensitivity that is used by a conventional OTDR, one example of which is disclosed in the '356 application. It is also an object of the present invention to nearly double the distance that can be monitored with the same laser power even if the distance is also increased by increasing the receiver sensitivity.